Hi silver,
I decided to try your map in single player, and here are some of the things that I found, and my settings:
Race chosen: Bloodsong
Difficulty: Easy
1) The creeps can still be blocked easily. Ok, normally they would bash their way through. But in this TD, the base towers (those towers that cost 5 gold, then upgrade up to volcano) have a LOT of hitpoints. So, the creeps would take a long time to kill them. However, if you decrease the tower hitpoints, the bosses would kill them easily.
2) I was on my way to passing in single player, on easy. I got to about lvl (30?) when I had to quit because there was a prac on in my lan

The problem is, if the map (any td map) is playable on single player, then it becomes ridiculously easy, as all the gold is concentrated in on area, so the player can build the most bad@ss towers. Its a weird balance bug.
3) As you say, I think the boss battles need work. Are the bosses only supposed to fight your heroes, or your towers also? I ended up making my hero run like !mad! to escape from the boss lol. Granted there are supposed to be 5 heroes, so I suppose that is realistic lol. I suppose I didn't really get the proper experience of it heh

Although I did have all the best stuff for my hero (damage items, lifesteal etc)
4) Perhaps consider making like a square tile or something at each of the pathing points, so that people actually know that there are points that the creeps must reach. It will make it a lot more player friendly (although they would soon enough figure it out, if they didn't sink first lol).
5) Those volcanoes and whirlwinds are still a bit big
Well, thats most of the bad stuff that I noticed. Best to get it out of the way lol. Otherwise, its a pretty good td. Its got rather cool spells/heroes, generally good towers and the boss idea/concept as implemented here is new to me. I also like the fact that the players start off with a hero, and the hero actually can get some nice items to make them effective
against creeps. I find it annoying to have to train a hero in TD's, after spending a lot to get the structure from whence they are trained.
Just a few ideas that I had when I was playing it...by no means complete
Cheers
Seralas